The C-nucleoside antibiotic, showdomycin, reacted rapidly with proteins of the L1210 leukemia, causing extensive plasma membrane damage and cell lysis. Both the extent of reaction with protein and cell damage were retarded by a reversible inhibitor of nucleoside transport, 6-((-hydroxy-5-nitrobenzyl)-thio)purine-9-beta-D-ribofuranoside, indicating that cell plasma membrane damage was directed only from the cytoplasmic side. Showdomycin irreversibly inhibited the nucleoside transport carrier.